For example, in an organic EL device 105 as shown in FIG. 3, light beams, which have been emitted from an EL light-emitting layer 102 laid on a reflective layer 101, are reflected on the interface between the EL light-emitting layer 102 and a seal layer 103 or between the seal layer 103 and the outside 104, leading to decrease in light-extraction efficiency.
Here, regarding the reflectivity of light on the interface at which refraction of the light occurs, when the interface is flat, the reflectivity depends on the incident angle of the light and the difference in refractive index between media which share the interface. For example, when the difference in refractive index therebetween is large, the reflectivity on the interface becomes high. Also, when light travels from the medium having a high refractive index to that having a low refractive index at an incident angle larger than the critical angle, 100% of the light is reflected.
Critical angle θc is the minimum incident angle of light at which the light is totally reflected when it travels from a substance having a high refractive index to that having a low refractive index, and expressed by the equation: θc=arcsin(n2/n1), where n1 denotes a refractive index of a substance through which light travels; n2 denotes a refractive index of a substance light enters; and n2<n1.
FIG. 4 is an explanatory view used for describing the above phenomenon. In this figure, reference numerals 111 and 112 respectively denote a first layer having a refractive index n1 and a second layer having a refractive index n2. Here, when light travels at an incident angle of critical angle θc with respect to a normal line (standard line) to the interface 110 between the first and second layers, the light is totally reflected on the interface 110 and thus, cannot be extracted from the second layer 112. In addition, light traveling at an incident angle of θx greater than critical angle θc with respect to the standard line is also totally reflected on the interface 110 and thus, cannot be extracted from the second layer 112.
Meanwhile, light traveling at an incident angle of θy smaller than critical angle θc with respect to the standard line transmits the interface 110 to be emitted from the second layer 112 to the first layer 111.
Light-emitting devices in which light is totally reflected when being emitted from a high-refractive-index medium to a low-refractive-index medium pose a problem in that the light-extraction efficiency is low.
In view of this, there have been proposed light-emitting devices having various structures, with which the light-extraction efficiency is attempted to be improved.
One proposed light-emitting device is an organic electroluminescence device including an anode, a cathode, one or more organic layers containing a light-emitting layer disposed between the electrodes and a diffracting grating or a zone plate, wherein the diffracting grating or the zone plate is disposed in position for preventing total reflection on the interface in the device (see Patent Literature 1).
Nevertheless, in the light-emitting device disclosed in Patent Literature 1, light emitted passes through low-refractive-index layers to reach the diffracting grating or zone plate and thus, limitation is imposed on prevention of total reflection.
Also, another proposed light-emitting device contains a concavo-convex patterned scattering layer at a back surface opposite to a light-emitting surface, wherein the scattering layer reflects/scatters light emitted through an intermediate layer from a light-emitting layer toward the light-emitting surface for light extraction (see Non-Patent Literatures 1 and 2).
One conventionally known light-emitting device, as shown in FIG. 5, includes a light-emitting layer 202 containing a light-emitting portion 204; an intermediate layer 205; and a fine concavo-convex pattern 206 in this order, wherein the intermediate layer and the fine concavo-convex pattern are laid over a second surface 203B of the light-emitting layer 202 which surface is opposite to a first surface 203A thereof.
However, such conventional light-emitting device has a light-emitting layer and an intermediate layer which are different in refractive index (for example, the refractive index n of the light-emitting layer: 1.8, and the refractive index n of the intermediate layer: 1.5) and thus, poses a problem in that the light-extraction efficiency from the light-emitting layer 202 is low due to total reflection.
Specifically, light beam 210a which is emitted from the light-emitting portion 204 to the second surface 203B of the light-emitting layer 202 and whose incident angle is θx greater than critical angle θc is totally reflected on the second surface 203, and cannot be extracted from the light-emitting portion 204.
Also, light beam 210b which is totally reflected on the interface between the light-emitting portion 204 and a seal layer 207 toward the second surface 203B and whose incident angle is θx greater than critical angle θc is totally reflected on the second surface 203B, and cannot be extracted from the light-emitting portion 204.
Furthermore, light beam 210c which is totally reflected on the first surface 203A of the light-emitting layer 202 toward the second surface 203B and whose incident angle is θx greater than critical angle θc is totally reflected on the second surface 203B, and cannot be extracted from the light-emitting portion 204.
In view of the above, demand has arisen for a light-emitting device which is improved in light-extraction efficiency.